The invention relates to a display device comprising an electro-optical display medium between two supporting plates, a system of picture elements arranged in rows and columns, in which each picture element is defined by two picture electrodes provided respectively on the facing surfaces of the supporting plates, and a system of row and column electrodes for driving the picture elements. The row electrodes are provided on one supporting plate and the column electrodes are provided on the other supporting plate. A system of switching elements are provided in which each switching element is incorporated between a row electrode and a column electrode in series with a respective picture element. Each switching element is formed by two series-arranged, oppositely-directed diodes.
Such a display medium is suitable for displaying alphanumerical information and video information by means of passive electro-optical display media, such as liquid crystals, electrophoretic suspensions and electrochromic materials.
The known passive electro-optical display media generally have an insufficiently steep threshold with respect to the applied voltage and/or have an insufficient intrinsic memory. In multiplexed matrix display devices these properties are the cause that the number of lines to be driven is small. Due to the lack of memory, the information presented to a selected row electrode via the column electrodes has to be written over and over again. Moreover, the voltages presented to the column electrodes are present not only across the picture elements of a driven row electrode, but also across the picture elements of all the other rows. Therefore picture elements, during the time they are not driven, experience an effective voltage which may be sufficient to bring a picture element to the on-condition. Furthermore, the ratio of the effective voltage which a picture element experiences in the on and off-conditions, respectively, decreases when the number of row electrodes increases. Due to the lack of a sufficiently steep threshold, the contrast between picture elements in the on- and off-condition decreases.
It is known that the number of rows which can be driven may be increased by integrating an extra switch per picture element. This switch ensures a sufficiently steep threshold with respect to the applied voltage and ensures that the information presented to a driven row electrode remains across a picture element during the time the remaining row electrodes are driven. The switch also prevents a picture element from experiencing an effective voltage during the time it is not driven.
A display device of a kind mentioned in the opening paragraph is disclosed in U.S. Pat. No. 4,223,308. This known display device uses as switching elements two series-arranged, oppositely directed diodes (back-to-back diodes). These switching elements are symmetrical with respect to a voltage zero, which is desired because liquid crystal display devices are operated with alternating voltage to prevent degradation of the liquid crystal in the case of direct voltage control. One of the two back-to-back diodes is always in the reverse direction so that the threshold voltage is determined by the breakdown voltage of that diode.
These known back-to-back diodes are formed by two p-n-junctions which are interconnected by means of a conductor. However, this structure has the disadvantage that the p-n structures have a voltage-dependent series resistance which is particularly large when one of the regions of the p-n structures is lightly doped, notably in amorphous and polycrystalline semiconductor materials. This means that upon energizing a picture element at the threshold voltage of the electro-optical effect, the voltage drop across the series resistance of the switching element has to be added in addition to the threshold voltage of the switching element, so that the resistance in the on-condition is unfavourably influenced and an unfavourable switching ratio is obtained. The switching ratio is the ratio between the resistance of the switching element in the on and off states.